Allow any files to be rendered as jinja templates

by including a comment ".. jinja" anywhere in the file. By convention,
this should be at the top.

os.getenv will use this so it can render a 'supported boards' list.

docs/rstjinja.py

@@ -1,6 +1,15 @@
 # Derived from code on Eric Holscher's blog, found at:
 # https://www.ericholscher.com/blog/2016/jul/25/integrating-jinja-rst-sphinx/
 
+import re
+
+def render_with_jinja(docname, source):
+    if "shared-bindings/support_matrix" in docname:
+        return True
+    if re.search('^\s+.. jinja$', source[0], re.M):
+        return True
+    return False
+
 def rstjinja(app, docname, source):
     """
     Render our pages as a jinja template for fancy templating goodness.
@@ -9,12 +18,12 @@ def rstjinja(app, docname, source):
     if app.builder.format not in ("html", "latex"):
         return
 
-    # we only want our one jinja template to run through this func
-    if "shared-bindings/support_matrix" not in docname:
+    # we only want specific files to run through this func
+    if not render_with_jinja(docname, source):
         return
 
     src = rendered = source[0]
-    print(docname)
+    print(f"rendering {docname} as jinja templates")
 
     if app.builder.format == "html":
         rendered = app.builder.templates.render_string(